The present invention relates generally to power generators, and, more particularly, to combustion turbine generators and methods for their starting.
Combustion turbine generators are widely used by electric power utilities to generate electricity. A combustion turbine power generator includes an electrical generator and a combustion turbine for driving the generator. The generator includes a rotor surrounded by a stator, electrical power being generated as the rotor turns within the stator. The combustion turbine drives the generator by turning a shaft connected to the rotor. The shaft is turned by an expansion of hot gas within the turbine. Air enters an inlet, is compressed by an air compressor, and then supplied to a combustor where fuel (e.g., natural gas) is burned to produce hot gas. The hot gas travels through the turbine where the expanding gas drives the shaft to turn the rotor.
The generator needs to be excited in order to create magnetic flux necessary for the generation of electrical power. An exciter, such as a brushless DC-field exciter, can provide the necessary excitation power. An exciter typically includes a rotating armature connected to the shaft and a field surrounding the armature. As the turbine turns the shaft, the shaft rotates the armature within the surrounding field to provide a current to the rotor.
A conventional combustion turbine power generator typically lacks a self-starting capability. Starting thus requires a motor or other external device to increase the rotation of the shaft up to a rotational speed at which the turbine can take over and drive the shaft.
A static start system can be employed for the static start of a conventional combustion turbine power generator. This technique employs a Static Frequency Converter (SFC) to provide three-phase ac power of variable frequency and magnitude to a generator stator while an excitation system simultaneously provides dc excitation to the field winding. The synchronous generator is operated as a synchronous motor. As the stator frequency is increased, the shaft accelerates to maintain synchronism, until ignition speed for the combustion turbine system is attained. Present static start systems use static excitation systems, employing slip rings and carbon brushes to provide dc excitation power. Another starting system is disclosed in one of the coinventor""s own patents, U.S. Pat. No. 6,285,089 B1, which has the same assignee as the present invention. The patent discloses an alternating current (AC) induction exciter that provides excitation to the rotor regardless of the rotational speed of the shaft and without the use of slip rings or brushes. Nonetheless, a conventional combustion turbine generator may not be equipped with the AC induction exciter.
It is generally thought that static starting is not possible with a generator that uses a brushless DC-field exciter unless a separate set of slip rings and brushes is used to provide excitation to the rotor during starting. It is costly, though, to install and maintain slip rings and brushes. It is costly, as well, to maintain a separate starting supply. Moreover, the use of a starting static excitation system introduces mechanical concerns related to vibration and operation.
With the foregoing background in mind, it is therefore an object of the present invention to provide methods for starting a combustion turbine power generator using a DC-field exciter and without slip rings or brushes.
This and other objects, features, and advantages in accordance with the present invention are provided by a method of starting a combustion turbine power generator by rotating the generator""s shaft to cause a shaft-driven DC-field exciter to supply all the DC power to the generator rotor field winding and by supplying electrical power to the generator""s stator with, for example, a static frequency converter. The result is an electrical torque on the shaft that increases the rotating speed of the shaft. The torque accelerates the rotation of the shaft thereby increasing field current supplied by the brushless exciter, thereby increasing the power supplied to the rotor, the increased power causing continued acceleration until the combustion turbine is rotating at a ignition speed.
More particularly, the combustion turbine power generator will include a shaft and a combustion turbine for driving the shaft. A turning gear may be used for rotating the shaft when it is not being driven by the turbine so as to effectively eliminate warping of the shaft. The combustion turbine power generator also typically includes a rotor connected to the shaft and a stator surrounding the rotor. A DC-field exciter comprising an armature connected to the shaft and a field surrounding the armature may also be part of the combustion turbine power generator.
The method may include configuring the combustion turbine power generator so that the rotor is connected to receive all DC electrical power needed for starting from the armature of the DC-field exciter. This configuring of the turbine power generator, accordingly, may include making the rotor to be devoid of brushes or slip rings, or simply not using the brushes or slip rings if the rotor already includes same.
The turning gear of the combustion turbine power generator may have a nominal rotational speed and an upper rotational speed. The method of starting the combustion turbine power generator may therefore include increasing the rotational speed of the shaft from the nominal rotational speed to the upper rotational speed. The turning gear may turn the shaft, thereby rotating the brushless DC-field exciter and causing it to generate a direct current in the generator rotor and a corresponding magnetic flux in the gap between the generator rotor and stator. As the speed increases, so does the field current and, hence, the magnetic flux.
Electrical power may be supplied to the stator as the shaft is rotating. The result is a stator current that also produces a corresponding magnetic flux. The magnetic flux from the rotor and the magnetic flux from the stator interact to create a torque on the shaft to increase its speed.
A field voltage may be applied to the DC-field exciter once the shaft reaches the upper rotational speed. The turning gear may then be disengaged. As the shaft rotates, electrical power may be supplied to the stator when the rotational speed of the shaft has reached the upper rotational speed.
The combustion turbine power generator may have a maximum start time. Accordingly, electrical power may be supplied to the stator so that the shaft reaches the starting rotational speed before the maximum start time. Current through the stator may give rise to an internal stator voltage. Accordingly, the electrical power also may be supplied to the stator so as to avoid an xe2x80x9cover-excitationxe2x80x9d or xe2x80x9cover-fluxingxe2x80x9d (i.e. excessive Volts/Hertz) condition of the stator, while still providing sufficient torque to accelerate or spin the rotor at a given speed, as appropriate.
Another aspect of the invention pertains to a combustion turbine power generator configured to implement the methods of starting already described. The combustion turbine generator may include a shaft, a combustion turbine for driving the shaft, a rotor connected to the shaft, a stator surrounding the rotor, and a DC-field exciter comprising an armature connected to the shaft and a dc field winding surrounding the armature. A turning gear may be used to rotate the shaft. The combustion turbine generator also may include a stator power supply that supplies electrical power to the stator during starting.
A starting controller may be connected to the turning gear and to the stator power supply. The starting controller may control the turning gear to rotate the shaft so that the DC-field exciter provides all DC electrical power to the rotor. The starting controller also may also control the stator power supply so that electrical power is supplied to the stator as the shaft is rotating.